In this manner the intersections, having the appearance of the teeth of a saw, will appear to advance with a uniform motion in the direction of the movement of oscillation, giving the appearance of a chain or row of pearls in motion in the case of a rectilinear row of cilia, or of a toothed wheel if the cilia are disposed in a circle.

Vide Dujardin, Hist. nat. des Infusoires.

(1130). We have already described the means whereby the Rotifera procure a supply of food, namely by exciting currents in the surrounding water; the materials so obtained pass at once into a pharynx, the relative capacity of which varies considerably in different species; from the pharyngeal receptacle it is conveyed into a singularly-constructed gizzard, to be bruised and broken down by an apparatus provided for that purpose; thus prepared, it is allowed to enter a third cavity, wherein digestion is accomplished, which may be called the stomach, and this, after becoming gradually constricted in its diameter, terminates at the caudal extremity of the body.

(1131). The usual arrangement of the digestive apparatus will be readily understood on reference to the annexed figures: thus, in Stepha-noceros Eichhornii (fig. 224), the pharynx (a) is very capacious, receiving readily the materials brought into it by the ciliated arms; the gizzard (e) is a small globular viscus, containing the instruments of mastication, hereafter to be noticed; while the digestive cavity properly so called (b), which presents no perceptible division into stomach and intestine, extends from the gizzard to the anal aperture.

(1132). In Brachionus urceolaris (fig. 225) the pharynx or oesophagus (e) is less capacious; the gizzard (f) exhibits through its transparent coats the peculiar dental organs placed within it; and the stomach (g) is seen partially folded upon itself by the retraction of the body. We observe, moreover, in this animal, appended to the commencement of the stomach, two large caecal appendages (h h), which were scarcely perceptible in the last figure, and which, no doubt, are of a glandular nature, furnishing some fluid to be mixed up with the bruised aliment contained in the stomach, to assist in the digestive process. To these secreting caeca, Ehrenberg has chosen to give the name of pancreas; but for what reason it is difficult to conjecture, since the first rudiments of a pancreas are only met with in animals far higher in the scale of animal existence; every analogy, indeed, would lead us to denominate these coeca the first rudiments of a liver - by far the most important and universal of the glandular organs subservient to digestion, and in a variety of creatures we shall afterwards find it presenting equal simplicity of structure.

In the Notommata centrum the caeca are merely two pouches opening into the top of the stomach, whereas in Notommata clavulata there are six of these appendages (fig. 230, e e) communicating with that enlarged portion of the digestive canal (c) which may be looked upon as the proper stomach.

(1133). We must now revert to the consideration of the dental apparatus contained in the gizzard, represented in situ in fig. 225,f, and exhibited on a still larger scale in fig. 228. This curious masticating instrument consists of three distinct pieces, or teeth, which are made to work upon each other by the contractions of the gizzard, so as to tear in pieces or bruise all matters made to pass through the cavity containing them. The central piece (fig.225,f) maybe compared to an anvil, presenting on its upper surface two flattened facets; and upon these the other two teeth (that might, without much stretch of fancy, be compared to two hammers) act. Each of the superior teeth may be described as consisting of two portions united at an angle: the larger portion, or handle as it might be called, serves for the attachment of muscles; whilst the other part is free in the cavity of the gizzard, and works upon the facets of the anvil, the edge being apparently divided into teeth resembling those of a comb, and evidently adapted to bruise or tear substances submitted to their action. Such is the transparency of the whole animal, that the effect of these remarkable organs upon the animalcules used as food is distinctly visible under a good microscope; and if the Rotifer be compressed between two pieces of glass, so as to break down the soft textures of its body, the teeth may, from their hardness, be procured in a detached state for minute examination.

The whole apparatus described above evidently resembles very closely the kind of stomach met with in the Crustacea, to which the Rotifera will be found gradually to approximate.

(1134). In Melicerta ringens the alimentary canal commences with a small oval orifice situated near the sinuated disk formed by the rotatory organs. It opens into an oesophagus that conducts the food down to the gastric teeth (fig. 227,1, e.) These, according to Professor Williamson's very excellent memoir upon the subject*, are implanted in a large conglobate cellular mass, which completely invests them. Their appearance, when highly magnified, is accurately represented in fig. 228; they consist of two essential portions - a pair of strong crushing plates, which bruise the food, and various appendages affording leverage and facilitating the action of the muscles upon them. The crushers are two broad elongated plates (fig. 228, a), each being about 1/800th of an inch long, and separated from each other at the mesial line, near which they become much thickened. From each of these plates there proceed laterally numerous parallel bars (fig. 228, b b), all of which are somewhat thickened at their inner extremities, where they are attached to the plates, whilst at their opposite ends they are united with the others of the same side by a curved connecting bar (fig. 228, c c), from the outer sides of which are given off various loops and processes. The three uppermost of these bars are the largest, the rest gradually diminishing in size as well as strength, till the inferior ones become almost invisible.